60 Dr J. J. Bigsby on the 



The copper is confined to the trap, as a universal rule. 



The north shore of Lake Superior is eminently trappose ; and 

 especially about Fort- William, where a region at least 120 miles 

 long consists of basalt, amygdaloid, porphyries, jasper, conglomerate, 

 and sandstone in the same mutual relations as on the south shore. 



The trap dikes, traversing granites and other crystalline rocks 

 indifferently, are a singular feature on the north shore, and abound 

 chiefly from Written llocks to the bottom of Michipicoton Bay. By 

 their dark and undeviating course through the gray, red, or green 

 rocks of the rugged coast, they strike the eye of the most incurious — 

 if only as ruined staircases, crossing bays and headlands and climb- 

 ing hills for miles. Their size, number, and direction are irregular. 

 They may be solitary, or twenty in company — sometimes all parallel 

 and close together. They often run with the general trend of the 

 coast.* 



Mr Logan divides them into three varieties, according as they are 

 homogeneous, syenitic or porphyritic. 



Professor Agassiz distributes the dikes of the whole lake into six 

 systems — each with its own mineral character and direction — its 

 own epoch of upheaval ; and each he announces to have been an 

 important agent in giving shape and direction to the district in which 

 it occurs. He truly says that the general outline of the lake is the 

 combined effect of many minor geological events taking place at 

 different periods. With some truth in it, this theory does not seem 

 to take into sufficient account the pre-existing metamorphic and 

 granitic rocks, and it overlooks the variety observed in the directions 

 of the dikes in the same neighbourhood. 



Dr B. stated that if he might be allowed to hazard an opinion, it 

 would be. that this curious assemblag-e of dikes — abounding as much 

 in the S. as on the N. coast — pervading all the crystalline rocks 

 indiscriminately, had ascended independently from the unseen, dis- 

 tant mass of trap beneath. They appear in many ways peculiar, 

 and have no visible connection with the traps he had been describing. 



Before the emergence of either traps or granites, Lake Superior 

 received its great outlines from the metamorphic rocks, — thrown 

 into their present position by still earlier upward movements : for 

 on the eastern half of both shores of the lake they strike E. and W. 

 with little variation ; while on the western half, these far extending 

 rock-masses strike WSW. and SW., — giving thus to the lake 

 a general eastward direction, with a gentle curve to the north, as 

 stated before. This done, Cambrian sandstone slowly took posses- 

 sion of the trough of the lake — just as we see a certain shell marl 

 is doing now. The anticlinal granites, which appeared afterwards, 



* Vide Quart. Journal of Roy. Inst. vol. win., p. 244. Bigsby on Lak< 



Superior. 



